1. Field of the Invention
The present invention relates generally to a fuel injection system for a multi-cylinder internal combustion engine having a fuel injection valve for each engine cylinder, and more specifically, in the fuel injection system, wherein air is introduced to the vicinities of injection holes of the fuel injection valves at times substantially synchronous with fuel injections, so as to facilitate atomization of the injected fuel.
2. Description of Related Art
In a fuel injection system which injects fuel into an induction system of an internal combustion engine via a fuel injection valve so as to control an operating condition of the engine, the facilitation of the atomization of the injected fuel has been strongly demanded so as to reduce injurious components involved in exhaust gas which are otherwise increased due to a deterioration of the combustion in the engine.
In order to satisfy this demand, so-called timed air mixture systems have been developed as disclosed, such as, in Japanese Second (examined) Patent Publication No. 57-54624. Specifically, in the induction system of the engine, the upstream side of a throttle valve is connected to the vicinity of an injection hole of a fuel injection valve via an air passage with an air control valve disposed therein. By opening the air control valve substantially in synchronism with fuel injection, air is supplied in the vicinity of the injection hole of the fuel injection valve at a time of the fuel injection so as to facilitate an atomization of the injected fuel.
Recently, in multi-cylinder internal combustion engines, the fuel injection systems of an individual-injection type have become predominant in consideration of the tightening of the emission gas regulation. In the individual-injection system, a fuel injection valve is provided for each engine cylinder, and a fuel injection is independently performed per engine cylinder. Accordingly, if the conventional timed air mixture system is applied to the individual-injection system as it is, the applied system includes the fuel injection valve and the air control valve for each engine cylinder.
When the air control valve is provided for each engine cylinder, since the air control valve can be opened synchronously with a valve opening of the fuel injection valve per engine cylinder, the air can be supplied only to the engine cylinder for which the fuel injection is being performed, so as to prevent the air from being wastefully fed to the other engine cylinders. Further, a time period corresponding to one cycle of the engine (720.degree. CA for a four-cycle engine) can be fully used for one opening-closing operation of the air control valve so that, even when an opening-closing response characteristic of the air control valve is poor, i.e. not so fast, an air supply timing and a corresponding air amount for each engine cylinder cam be controlled to a sufficient level.
On the other hand, when the air control valve is provided for each engine cylinder, the number of the air control valves and corresponding drive circuits have to be increased according to the increased number of engine cylinders, and further, each air passage has to be significantly strong enough for mounting the air control valve therein.
Accordingly, when the conventional timed air mixture system is applied to the individual-injection system as it is as described above, the applied system becomes complicated with an increased weight so that the applicability of the system onto a vehicle is deteriorated.
On the other hand, in order to solve the above-noted problem, it may be arranged that a single air control valve is provided at a common air inlet side of the air passage, i.e. upstream of a branch point of the air passage so that the air is fed to all the engine cylinders at one time by opening the air control valve. With this structure, since the single air control valve is required irrespective of the number of engine cylinders, the applicability of the system onto the vehicle is not deteriorated.
However, since the air is fed to all the engine cylinders by opening the air control valve, the air is wastefully supplied to those engine cylinders for which the fuel injection is not performed. As a result, it may happen that, for example, an engine speed can not be sufficiently suppressed during the engine idling, or the engine brake becomes ineffective during deceleration of the engine to deteriorate the deceleration feeling.
On the other hand, apart from the foregoing problems, it has been confirmed by the present inventors that satisfying the following conditions can facilitate the atomization of the injected fuel:
(1) unifying supply timing and supply amounts of the air distributed to the fuel injection valves for the engine cylinders to equalize combustion states among the engine cylinders; PA1 (2) suppressing a pressure loss of the air in the air passage to ensure a high supply pressure of the air; and PA1 (3) lowering a supply pressure of the air rapidly after the fuel injection to prevent wasteful consumption of the air.
However, in the conventional fuel injection systems, consideration has not been paid to satisfying the foregoing conditions.